The design of heat transfer systems for applications in aircraft and other dynamic contexts involves stringent constraints on weight, form factor, breadth of operating conditions, and robustness of operation. Conventional heat exchangers based on convective heat transfer face a number of challenges for these applications: the need for dedicated, active pumps to drive flow; the requirement of large volumes of the working fluid due to the inherently poor efficiency of sensible heat transfer, and the requirement of large temperature differentials to drive significant rates of transfer.
Heat pipes are an attractive alternative to conventional heat exchangers. Heat pipes utilize evaporative cooling to transfer thermal energy from a heat source to a heat sink by evaporation and condensation of a working fluid. Evaporative cooling has the capability to remove up to ten times the thermal energy of an equivalent volume of liquid by sensible cooling (e.g., circulating coolant loop). A typical heat pipe includes a sealed pipe containing a quantity of working fluid and a capillary wick arranged along the inner wall of the pipe. As one end of the heat pipe is exposed to the heat source, the working fluid in that end draws thermal energy from the heat source and vaporizes, increasing the local vapor pressure in the tube. The latent heat of evaporation absorbed by the vaporization of the working fluid reduces the temperature at the hot end of the pipe. The vapor pressure over the working fluid at the heat source side of the pipe is higher than the equilibrium vapor pressure over the condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapor condenses, releases its latent heat, and warms the cool end of the pipe. The condensed working fluid, now a liquid, is transferred back to the heat source by the capillary wick.
Recent advancements in heat pipe fabrication have resulted in microfluidic heat pipes for very small applications, such as for cooling microelectronics. Thin, planar heat pipes have also emerged as a leading technology to cool circuit boards, laptop computers, or other applications having height restrictions. In one example, a microfluidic heat pipe structure is etched into a silicon wafer using conventional microchip fabrication techniques. Capillary channels etched into the structure are augmented with wicking material to provide a means to return condensed working fluid back to the evaporator.
Other heat pipe structures include porous valve metals disposed between the liquid/vapor interface of the evaporator. The porous valve, typically made of a sintered powdered metal, has interstitial voids that act as capillaries to wick the working fluid through the porous metal as the working fluid evaporates.
One of the primary challenges faced by heat pipe designers is assuring the wick provides positive liquid flow from the condenser region to the evaporator region. The pumping capability of the wick is adversely affected by height (operation against gravity) and length (mass flow resistance). Careful design consideration must be given to the amount of heat that must be removed via evaporative cooling and assuring an adequate supply of working fluid to accomplish the heat removal. In microfluidic heat pipe applications, capillary channels and wicking structures are typically utilized to accomplish this purpose. However, the wicking structure must generate sufficient capillary force to assure positive liquid flow.
One drawback noted with current heat pipes is that the capillary wicking force, either in the capillaries or in the wicking material, is not always sufficient to overcome the dynamic forces that may be introduced to the system. Current wicks generate only a fraction of one atmosphere (<1 atm) of pumping pressure. This small pressure difference is easily overwhelmed by gravity or by inertial forces (e.g., acceleration along the axis of the wick). In the presence of these external forces, the heat pipe is prone to failure due to dry-out of the evaporator. For example, the design of heat pipe structures in aerospace applications is particularly challenging. The evaporator and condenser sections may need to be spaced more than 1 meter apart for proper thermal differential. Additionally, the aircraft may develop dynamic forces of acceleration that may exceed three times the force of gravity (3 g). In extreme situations, such as when aerospace vehicles travel at or near the edge of space, the dynamic loads may be as high as ten times the force of gravity (10 g). In these situations, a wicking structure is required that will overcome more than 1 atmosphere (0.1 megapascals) of pressure head. There are no known wicking structures that will generate sufficient wicking forces to overcome static and dynamic loads of this magnitude.